Looking Up: Find the North Star

Thursday

Feb 14, 2013 at 3:13 PMFeb 14, 2013 at 3:13 PM

By Peter Becker

Managing Editor

One of the most famous stars in the night sky, as seen from the northern hemisphere, is Polaris, the North Star. It marks the end star in the handle of the Little Dipper- which is actually the constellation Ursa Minor the Little Bear. Interestingly, the star has not always been the "North Star."

It is the one easily visible star that never seems to go anywhere. As the world turns (no, not the soap opera), the constellations of stars appear to rise in the east and set in the west, in the same way the Sun makes its daily trek in the daytime sky (ever see the Sun in the night sky?). Constellations and stars that never set are referred to as "circumpolar." They circle a point in the northern sky known as the North Celestial Pole, right next to Polaris.

Your latitude on the globe is easily determined by noting how high above the north horizon, the North Star shines. On the ocean from the Earth’s equator, the North Star would be right on the horizon; at the North Pole, it would shine straight overhead.

To find the North Star, look due north (opposite where the Sun is at noon time), and about half way up in the sky (assuming you live in mid-northern latitudes). On a late February evening, you will see the familiar Big Dipper riding high in the northeast. The two stars in the front of the Dipper’s "bowl" appear to point right at the North Star.

Polaris just happens to lay very close to the North Celestial Pole, making it seem to be a pivot for the rest of the sky to circle around. It wasn’t always so. The Earth’s axis of rotation has an extremely slow wobble, making a circle on the sky every 25,800 years. An immense amount of time by human standards, it makes a difference only when looking back or forwards at least hundreds of years. The Earth’s axis changes where it is pointing in a circle 47 degrees wide. If you go back 3,000 years, the North Celestial Pole lay very near the star Thuban in the constellation Draco the Dragon. Thuban is easily seen though a bit dimmer than today’s North Star. Thuban was the north star when the Egyptians were building the pyramids, and in fact built the pyramids to align with Thuban.

Through the ages the North Celestial Pole spends great lengths no where close to any star easily visible to the unaided eyes. We are fortunate to live in an age when we do have a bright star to mark the north. In the past 500 years, when seafarers were exploring the globe, Polaris was reasonably near the North Celestial Pole, just in time to aid their navigation when mankind needed it the most.

The slow wobble of the Earth’s axis is caused by gravitational tugs from the Sun and Moon, pulling on the Earth’s uneven shape (it is a bit wider at the equator). The wobble, known as the "Precession of the Equinoxes," was first realized by ancient Greek astronomer Hipparchus, around 130 B.C.

The same situation happens "Down Under" as well. Southern hemisphere residents, however, do not have a bright "South Star" during this epoch. The closest naked eye star to the South Celestial Pole is Sigma Octantis. This star is magnitude 5-1/2, just visible without optical aid, in a dark sky. Polaris is 2nd magnitude.